The field of the invention is radiology and particularly medical imaging and more particularly a method and system or apparatus for X-ray fluoroscopy. An embodiment of the invention relates to a method and an apparatus for attenuating or reducing noise in images acquired by means of a fluoroscopic X-ray apparatus or system.
In the field of medical imaging, it is well known to utilize fluoroscopic images to aid guidance of surgical instruments during an operation or surgical procedure. An operation or surgical procedure guided by fluoroscopic images is thus typically performed by passing a catheter inside the vascular network of a patient, while a contrast agent has been earlier injected into this vascular network so as to opacify the network and improve visibility of the vessels. The progress of the operation or surgical procedure is then followed by X-ray imaging. Such an operation or surgical procedure is generally relatively long, and it is thus necessary to limit the dose of X-rays to which the patient is subjected, in order to avoid causing lesions or burns to the tissue due to radiation. But due to this limitation of the dose of X-rays, fluoroscopic images obtained comprise a relatively high level of noise, likely to impair visibility of the operating instruments.
Filtering techniques are generally employed to decrease quantic noise present in these images. Techniques known under the name of Fluoroscopic Noise Reduction (FNR) is thus generally carried out on images acquired using the X-ray fluoroscopic system. The objective of this FNR processing is to filter the noise present in the image while maintaining the contrast of information present in the images. FNR processing is typically performed by applying a time filter to the regions of the images where there is no motion. Detection of the motion based on individual variation in the intensity of each of the pixels considered separately is then carried out to determine the existence or the absence of motion in the image. A pixel is thus considered as being in motion when its inter-image intensity variation exceeds a threshold linked to the standard deviation of the noise. These pixels determined as being in motion are not, or only slightly, filtered. However, processing via recursive time filtering is applied to the pixels determined as being fixed. U.S. Pat. No. 6,314,180 discloses an example of such FNR processing.
Such FNR processing nevertheless has certain disadvantages. In the event where motion detection is not corrected, fixed pixels, noisy but detected as being in motion, will remain noisy. In addition, the time filter will combine the pixels in motion that it falsely considers as being fixed with their preceding location. This combination risks consequently causing a reduction in contrast of these pixels and can even introduce phantom objects in the filtered image. In addition, due to the fact that the pixels determined as being in motion are not, or at the very least, filtered, so as to limit this phenomenon of shift, a group of pixels changing location will be followed by a tail of noisy pixels at the place where the group of pixels in question was located in the preceding image in the sequence of fluoroscopic images.